1. Field of the Invention
The present invention relates to a motor driving technique, and particularly to a technique for controlling the starting of a motor.
2. Description of the Related Art
There is a widely used technique in which the conduction period (ON time) for each coil of a motor is controlled according to a pulse signal such as a pulse width modulation signal or the like, in order to rotationally drive a single-phase motor, or a multi-phase motor, with a desired torque.
Let us consider a case of starting to drive a motor that has stopped. In this case, in a case that a switching voltage is suddenly applied to each coil of the motor, with a pulse width that corresponds to a desired target torque, a rapid increase occurs in the current flowing through the coil of the motor. When the motor is started, and, in particular, in a case that the revolution is close to zero, the motor provides no power generation function. Accordingly, in this case, the value of the current flowing through the coil is obtained by dividing the voltage thus applied by the winding resistance. On the other hand, the resistance value of each coil is set to an extremely low value, which is designed in  order to reduce the power loss. Accordingly, in some cases, with such an arrangement, the coil current exceeds the rated current of the driving circuit or the rated current of the coil itself, leading to the reliability of the circuit being adversely affected.
On the other hand, the counter electromotive voltage generated by each coil is proportional to the rate of change in the current flowing through the coil over time. Accordingly, in a case that a rapid increase occurs in the coil current as described above, an extremely large counter electromotive voltage is generated, which can exceed the rated voltage of the driving circuit.
For such a reason, soft start control is employed in which current control is performed such that the amount of current flowing through the coil is gradually increased immediately after the motor is started (see Patent documents 1 and 2, for example). With the soft start control described in Patent document 1, first, a soft start voltage is generated, the voltage value of which is gradually increased over time. Then, a comparison is made between the soft start voltage thus generated and a triangle wave or sawtooth wave cyclic voltage so as to generate a pulse modulation signal with a duty ratio that gradually increases over time. Such an arrangement provides the conduction period that gradually increases over time for each coil, thereby offering the soft start control.
[Patent Document 1]
Japanese Patent Application Laid-Open No. Hei 07-95792
[Patent Document 2]
Japanese Patent Application Laid-open No. 2001-45790